A perculator (?perc? for short) is basically an additional water chamber found in the main shaft of the bong tube. Several companies use different designs, but they all focus on more water to smoke interaction, making for a better filtered and cooler hit. Some of the better known companies take it a step farther by adding diffusers into their percs, which basically break the smoke up into smaller bubbles making a cooler smoke.

Here is an example of a 3-chambered Phx trinity.




What is a diffuser?
A diffuser is another type of downstem for a bong. Contrary to a downstem, a diffuser resembles a test tube as its sealed on the end or pierced with many small holes or slits. These slits break up the smoke into smaller bubbles than a normal downstem.

Why the need for smaller bubbles? The smaller the bubbles, the greater the surface area of the smoke that contacts the water, thus filtering and cooling the smoke more efficiently. This makes the bong almost silent silent. Many glass companies sell diffusers to fit their bong: Roor, Phx, and Bluedot.

Here is a pic of a german Roor diffuser along next to a Phx diffuser.